This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to hydraulic fill fluids used in pressure transmitters.
Process transmitters are used to remotely monitor process variables of process fluids. For example, pressure transmitters are commonly used in the chemical processing industry for sensing pressures of petrochemicals or water. A pressure transmitter includes a sensor or transducer that produces an electrical output in response to physical changes in a process pressure. For example, a capacitive pressure transducer produces an electrical signal based on a change in capacitance due to a change in the pressure of a process fluid. The electrical signal of the sensor is processed by the transmitter circuitry to produce an electrical output that can be monitored as an indication of the pressure of the process fluid. Pressure transmitters also include electronics and circuitry for either remotely monitoring the electrical output through a control loop or network at a central monitoring location such as a control room, or locally such as with an LCD screen.
A typical capacitive pressure transducer includes a simple hydraulic system for communicating the process fluid pressure to the capacitive transducer. The hydraulic system is comprised of a hydraulic passageway that is filled with a precise level of fill fluid. At a first end of the hydraulic passageway is a transmitter isolation diaphragm that separates the fill fluid from the process fluid. At a second end of the hydraulic passageway is a sensor diaphragm that serves as a variable capacitor plate for the pressure sensor. The fill fluid typically comprises a hydraulic fluid that conveys the process fluid pressure from the transmitter isolation diaphragm to the sensor diaphragm. As the process fluid pressure fluctuates, the process fluid exerts a corresponding force on the isolation diaphragm at the first end of the hydraulic system, which through the fill fluid adjusts the position of the sensor diaphragm.
Additionally, remote seal systems are often used to distance the pressure transmitter from hazardous measurement environments, or for linking the pressure transmitter with inconveniently located process fluids. For example, remote seals are often used with corrosive or high temperature process fluids. In those situations, a remote seal having a capillary tube filled with hydraulic fill fluid can be used to relate the pressure transmitter to the process fluid while the pressure transmitter is located a safe distance away. The capillary tube may extend tens of meters in order to couple the pressure transmitter with the process fluid. The remote seal includes an isolation diaphragm at a first end of the capillary tube that separates the fill fluid from the process fluid. The second end of the capillary communicates with the isolation diaphragm of the transmitter. The incompressible fill fluid thus translates a pressure change of the process fluid from the remote seal isolation diaphragm to the transmitter isolation diaphragm. Thus, the electrical output of the pressure sensor is directly related to qualities of the hydraulic fill fluid, such as volume and capacitance, in the hydraulic system and the remote seal.
Process transmitter passageways and remote seal capillary tubes are filled at the factory with a precise amount of hydraulic fill fluid and then sealed. System performance is correlated with the precise level of fill fluid and is degraded with temperature fluctuations. For example, pressure transmitters are frequently used in high temperature settings. Fill fluids operating in such environments undergo thermal expansion that asserts forces against the various isolating diaphragms, which causes the pressure sensor to read a “back pressure,” thus interfering with obtaining accurate pressure readings from the process fluid. Additionally, the capacitance of capacitive pressure sensors is affected by the presence of the hydraulic fill fluid between the capacitor plates of the sensor. Therefore, there is a need for hydraulic fill fluids having improved operating characteristics and qualities.